The invention applies more particularly to manufacturing an alternator whose rotor includes superconducting inductor windings, i.e. made of a substance which becomes superconducting when it is cooled to a sufficiently low or "cryogenic" temperature and equal, for example, to 4.degree. K. In addition to these windings and the electrical connection means necessary to power them, the rotor also includes a cooling circuit which is capable of cooling the windings from ambient temperature to said cryogenic temperature before the alternator is brought into action, and thereafter of maintaining said windings at that temperature. Said circuit includes a refrigerator machine, e.g. a helium liquefier. The lower the temperature to be reached, the higher the cooling power necessary for maintaining superconductivity, i.e. the power which must be supplied to the refrigerator machine, and, for a given temperature, that power is proportional to the heat lost from the rest of the apparatus to the cryogenic windings, i.e. to the heat flow which must be removed.
To reduce said heat flow, the rotor is separated into a plurality of zones including:
a "hot" zone which remains substantially at ambient temperature during operation and whose principal component is the shaft which provides mechanical rigidity of the rotor as a whole while allowing it to rotate between two bearings;
a "cold" zone which may be called the inductive zone because it includes the windings; said cold zone is brought to cryogenic temperature during operation; and
thermal transition members which provide mechanical connection between the shaft and the inductor, i.e. which carry the inductor by bearing against suitable support structures forming part of the shaft, the temperature of these members in operation increasing from a cold end adjacent the inductor to a hot end adjacent the support structure.
Heat losses due to the temperature difference between the cold zone and the neighbouring hot zone result from radiation and from conduction, a vacuum being provided to prevent heat loss by convection. Highest heat losses are due to conduction. They decrease with the thermal resistance between the two zones. What is meant here by "thermal resistance" of a component with a cold end and a hot end is the ratio of the temperature difference between these two ends divided by the rate of heat flow along the component from the hot end to the cold end. Said thermal resistance can be measured in degrees Kelvin per watt (K/W).
When very low temperatures such as 4.degree. K. have to be maintained in large spaces the necessary cooling power becomes very expensive and it is sought to increase the thermal resistance between the cooled zone and the non-cooled zone. For this purpose, heat insulating materials are used. As for the transition members which provide mechanical connection between these zones, their thermal resistance is increased by increasing their length and reducing their cross-section. However, the thermal resistance which can thus be provided is not as great as is required unless mechanical strength is excessively reduced.
To obtain both suitable mechanical characteristics and minimum heat losses, the shaft is made in the form of: a large-diameter rigid tube which surrounds the inductor without coming into contact therewith. Two circular discs close respective ends of the tube and two hollow shaft ends of smaller diameter than the tube, projecting axially from has the centers of respective ones of the discs engage in bearings. The inductor has the shape of a sealed drum located inside the rigid tube and extended at each end by a thin tube of the same diameter as the sealed drum, said tube constituting a thermal transistor member. Each transition tube bears against a respective support structure which is integral with the shaft and is located in the vicinity of a respective one of the end discs. The assembly has a cylindrical structure about the axis of rotation of the rotor.
The fact that the inductor is subject to temperature variations which are very different from those to which the shaft is subject causes longitudinal movements of the ends of the inductor with respect to the support structures, especially at the beginning and at the end of the periods when the inductor is kept cold. It is possible for these relative movements to be made without damage by elastic deformation of the support structures. However, these structures, like the thermal transition tubes, must be able to transmit all of the rotor drive torque to the inductor. The use of elastic deformation therefore entails the use of large parts. This appreciably increases the dimensions and the cost of the rotor.